static-if-inline

The ilist patch from Andres Freund introduces a cute trick for defining
maybe-inline functions, which works regardless of whether the compiler
supports inlining, and eliminates the need to write the code twice
(first in the header and also the .c file.)  It's really quite a simple
thing, but the whole topic on inlining has generated a lot of debate.

So to remove that controversy out of said patch, here's a preliminary
one: get rid of duplicate definitions in sortsupport.c, list.c, mcxt.c
(which are currently defined as static inline in their respective
headers for compilers that support it, and as regular functions in the
.c files for those that don't).

What's being done in this patch is exactly what would be done in the
ilist code.  So if there are problems with this, please speak up.

--
Álvaro Herrera                http://www.2ndQuadrant.com/
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